A light-weight magnesium alloy exhibiting excellent specific strength and specific rigidity has been studied as a constituent material for various structural members, e.g., a housing, of mobile electric and electronic devices, such as, cellular phones and laptop computers. As for structural members formed from the magnesium alloy, cast materials (for example, the AZ 91 alloy based on the American Society for Testing Materials Standard) by a die casting process or a thixomold process are the mainstream. In recent years, a structural member produced from a sheet, which is formed from a magnesium alloy for elongation typified by the AZ 31 alloy based on the American Society for Testing Materials Standard and which has been subjected to press forming, has been used.
PTL 1 discloses that a rolled sheet formed from the AZ 91 alloy or an alloy containing Al to the same extent as the AZ 91 alloy is produced under a specific condition and the resulting sheet is subjected to press forming.
PTL2 discloses a technology to produce a cast material serving as a raw material for such a rolled sheet with a twin-roll type continuous casting apparatus. The twin-roll type continuous casting apparatus is an apparatus to obtain a sheet cast material by feeding a molten material to between a pair of casting rolls rotating in directions opposite to each other and quenching and solidifying the molten material between the casting rolls. The cast material produced with this twin-roll type continuous casting apparatus is usually coiled on a take-up reel after being formed through rolling and the like, and is carried to another secondary forming site on a take-up reel basis or is shipped to a customer.
PTL3 discloses a casting nozzle suitable for a twin-roll type continuous casting apparatus. This nozzle is formed by combining a pair of main body sheets disposed discretely and rectangular parallelepiped side dams disposed on both sides of the two main body sheets, and an opening portion is rectangular.
Among the magnesium alloys formed by the above described technologies, magnesium alloys having high strength and exhibiting excellent corrosion resistance, flame retardancy, and the like have large contents of additive elements. For example, in the case where cast materials are compared, the AZ 91 alloy having a content of Al larger than that of the AZ 31 alloy has high tensile strength and excellent corrosion resistance as compared with the AZ 31 alloy. Furthermore, regarding magnesium alloys having the same composition, in general, the strength of a formed material, which is produced by subjecting a cast material to various types of plastic forming, e.g., rolling, forging, drawing, or pressing, is higher than the strength of the cast material.